title: Growth and interaction of the Tetronic 904 micelles in aqueous alkaline solutions.
abstract: The influence of sodium hydroxide (NaOH) on the aggregation characteristics of the Tetronic 904 (T904) has been studied in the aqueous medium by dynamic light scattering (DLS), small angle neutron scattering (SANS), and viscometry methods. This polyethylene oxide (PEO)-polypropylene oxide (PPO) based X-shaped triblock copolymer shows pH sensitive aggregation characteristics due to the presence of a central amine group attached with the PPO block. The aqueous micellar solutions of this copolymer have been found to show a large increase in the room temperature (30 °C) relative viscosity with an increase in NaOH concentration, before they undergo phase separation at 1.25 M NaOH concentration. SANS and DLS studies ascribe this behavior to a sphere to rod growth of the copolymer micelles on approaching the cloud points of the copolymer solutions. DLS studies show that the observed micellar growth is accompanied by critical scattering due to the onset of an attractive intermicellar interaction. To understand the role of NaOH in inducing the observed micellar shape transition, we have also studied the effect of NaOH on the micellar solutions of Pluronic P84, which does not show pH sensitivity due to the absence of the central amine group but has the same weight fraction of the PEO block (40%) as that of T904. Quite interestingly, the P84 micelles too have been found to undergo a similar room temperature sphere to rod shape transition in the presence of NaOH. The observed growth and interaction of the Tetronic micelles have thus been attributed to the dual effects of NaOH in influencing the aggregation characteristics of the Tetronic molecules. On one hand, it stabilizes these micelles by the deprotonation of the copolymer molecules, and on the other hand, its dehydrating effect on the copolymer molecules promotes the onset of micellar growth and intermicellar attractive interaction at the room temperature. The observed results are the first of their kind in the aqueous Tetronic system.
authors.affiliation: Chemistry Division, Bhabha Atomic Research Center, Mumbai 400 085, India. rajibg@barc.gov.in
authors.forename: R
authors.initials: R
authors.lastname: Ganguly
authors.forename: Y
authors.initials: Y
authors.lastname: Kadam
authors.forename: N
authors.initials: N
authors.lastname: Choudhury
authors.forename: V K
authors.initials: VK
authors.lastname: Aswal
authors.forename: P
authors.initials: P
authors.lastname: Bahadur